Early retroreflective sheeting had an exposed-lens construction, but its retroreflective light was blanked out when the lenticular surface of the exposed lenses was covered with water. This problem was answered by enclosed-lens retroreflective sheeting in which, as taught in U.S. Pat. No. 2,407,680 (Palmquist et al.), the lenses were embedded within the sheeting which had a flat, transparent cover film. This allowed incident light rays to be focused onto the specularly reflective layer irrespective of whether the front of the sheeting was wet or dry. In U.S. Pat. No. 3,190,178 (McKenzie '178) the same problem is solved in a different way, namely, by modifying retroreflective sheeting of the exposed-lens type wherein lenses are partially embedded in a binder layer. In McKenzie '178, the exposed lenses are protected by a cover film to which the binder layer is sealed along a network of interconnecting lines, thus forming a plurality of hermetically sealed cells within which the lenses are encapsulated and have an air interface. Such exposed-lens sheeting is called "encapsulated-lens retroreflective sheeting".
In the method taught in McKenzie '178 for making encapsulated-lens retroreflective sheeting: (1) substantially a monolayer of lenses, such as glass microspheres, is embedded into a carrier web to a depth not exceeding 50% of the diameter of each microsphere, (2) specularly reflecting material is deposited over the lens-bearing surface of the carrier web, (3) a solution of binder material is applied over the specularly reflective deposit, (4) after drying the binder, the carrier web is stripped off leaving the lenses partially embedded in the binder,(5) a cover film is laid over the exposed lenses, and (6) heat and pressure are applied along a network of interconnecting lines causing the binder material to soften and flow around the lenses into contact with the cover film, thus forming the aforementioned hermetically sealed cells. The binder material typically includes a white pigment such as TiO.sub.2 to give the sheeting a whiter color as well as a cleaner color in any area to which another color has been applied by silk screening. The color of the sheeting as well as the adhesion to a top film is enhanced if the specularly reflective material, usually aluminum, between the lenses is carried away by the carrier web.
Early binder layers typically were composed of a high molecular weight linear thermoplastic acrylate and a pigment. In U.S. Pat. No. 4,025,159 (McGrath '159), the durability of the encapsulated-lens construction was improved by curing the binder after bonding the cover film and base sheet together. U.S. Pat. No. 4,653,854 (Miyata) discloses attaching pendent hydroxyl groups to the backbone of the acrylate polymers used for the bead bond. Incorporation of polyisocyanates into the formulation allowed for crosslinking of the binder through the formation of urethane linkages.
While these developments helped address issues related to product durability and manufacturing, they generally required large amounts of solvent for coating operations. Due to cost and environmental considerations, this is a drawback in the manufacture of retroreflective sheeting.
Efforts to implement solventless binder technology into the construction of cellular retroreflective sheeting generally consisted of dissolving a high molecular weight acrylate polymer in one or more reactive diluents and then coating the material warm. This technique, however, also has certain drawbacks, such as the requirement of chilling to low temperatures for bead stripping. Belisle et al., in U.S. Pat. No. 4,721,649 reported using a solventless, thermoformable two-component urethane as a polymeric binder layer for "embedded-lens" retroreflective sheeting. U.S. Pat. No. 4,897,136 (Bailey et al. '136) and U.S. Pat. No. 5,064,272 (Bailey et al. '272) report using a thermoplastic urethane or an olefinic polymer (i.e. ethylene methacrylic acid) to develop a solventless binder for flexible cellular retroreflective sheeting.
A solvent-based semi-interpenetrating polymer network composition for use in embedded lens retroreflective sheeting is described in U.S. Pat. No. 5,008,142 (Wilson et al.)